I see nothing wrong with "it is one of 50 known" if the seller knows this -- meaning, if it's a chip such as a C&S, Ivory or MOP... or any other category where usually those collectors of that specialty have a pretty good handle as to what's in each others' collection to share with those who read the description... I also feel that it doesn't hurt to mention what you know about the chip's worth (value); meaning if you know the last 10 examples sold for $60 it sure wouldn't hurt to mention it...

Regarding book value; I still look at book value as simply being a guide, but by mentioning a chip's performance you are simply mentioning actual fact (...and yes, I do agree that anything can change at any minute in this hobby, but not that often to not bring up actual recorded fact).